1. Field of the Invention
The invention relates to superconductive circuits particularly with respect to superconductive tunnel junction devices.
2. Description of the Prior Art
Superconductive tunnel junction devices are known in the prior art that comprise first and second superconductive electrodes with a tunnelling barrier therebetween. Such devices are typically fabricated by forming a lower superconductive layer on a substrate in a vacuum chamber. The vacuum system is then disrupted and the substrate removed for patterning the lower superconductive layer. A junction-defining insulating layer may also be formed on the lower superconductive layer. The wafer is then returned to the vacuum chamber and after pump down, a sputter etching step performed, often in an argon plasma for cleaning the surface of the superconductive material. The barrier layer is then formed and the remainder of the device completed by known steps.
In these prior art procedures, the lower superconductive electrode is severely contaminated before barrier formation is initiated. The contamination arises from such sources as the use of photoresistive material to pattern the lower superconductive electrode, photoresistive lift-off chemicals, photoresistive stripping agents such as liquid or plasma, photoresistive patterning of junction-defining insulator windows in, for example, a silicon oxide insulating layer, particularly in the instance of refractory material device, and, contact of the lower superconductive electrode surface with laboratory air. Any of these sources may produce significant contamination for the subsequent formation of tunnelling barriers, where even one atomic layer can substantially affect the properties of the tunnel junction device.
Therefore, it is expected that when utilizing these prior art processes, a sputter etch cleaning of the lower superconductive electrode surface would be performed before formation of the barrier.
Such prior art is discussed in Applicants' Assignee's U.S. Pat. No. 4,176,365, issued Nov. 27, 1979. Further examples of such prior art are discussed in an article in the IEEE Transactions on Electron Defices, Volume ED-27, No. 10, October 1980, on page 1998 entitled "Niobium Oxide-Barrier Tunnel Junction" and in the IBM Journal of Research and Development, Volume 24, No. 2, March 1980 on page 212 entitled, "Fabrication and Properties of Niobium Josephson Tunnel Junctions".
In these prior art techniques, it has been observed that depositions and removals of insulator layers, even if followed by a sputter etch cleaning step, can alter the surface of a supeconductive electrode so as degrade tunnel junction properties.
Another example of a prior art procedure for fabricating superconductive tunnel junction devices is found in Applied Physics Letters, Volume 39, No. 3, Aug. 1, 1981, in an article on page 280 entitled "Selective Niobium Anodization Process for Fabricating Josephson Tunnel Junctions". In this process, the lower superconductive electrode is formed on a substrate in the vacuum system. The barrier layer is then formed on the lower superconductor and the upper superconductor is formed on the barrier, in situ, without breaking the vacuum system and removing the substrate therefrom. Since this sandwiched configuration is formed without breaking the vacuum system, the surface of the lower superconductive electrode which forms the interface batween that electrode and the barrier ramains uncontaminated and therefore it saw believed with respect to this prior art process, that a sputter etch cleaning step of the surface of the lower superconductive electrode would not perform a useful function. In an altenative version of this process, the vacuum system may be disrupted after the in situ formation of the barrier and the substrate removed from the vacuum system for a thermal oxidation treatment. Thereafter, the upper superconductive electrode is formed once again in the vacuum system. With this variation, the interface between the lower superconductive electrode and the barrier remains uncontaminated thereby again obviating cleaning the surface of the lower superconductive electrode prior to barrier deposition. Further details of this process may be found in Applicant's Assignee's U.S. patent application Ser. No. 179,311, filed Aug. 18, 1980 in the name of Harry Kroger entitled "Superconductive Tunnel Junction Device and Method of Manufacture" now U.S. Pat. No. 4,421,785.